The present invention relates to an EMI (ElectroMagnetic Interference) shield technique and, more particularly, to an EMI shield filter to be attached to the screen of a display apparatus and a display apparatus having this EMI shield filter.
The EMI standards such as CISPR22 recommended by CISPR (abbreviation of French representation for International Special Committee on Radio Interference) regulate the radiation amounts (limit values) of the vertical and horizontal polarized waves of the EMI radiated from the equipment.
An EMI shield filter must have a high electromagnetic shield performance against electromagnetic waves radiated from a display apparatus and the like on the basis of the standards, and good light transmission properties as a display apparatus.
A display apparatus, e.g., a plasma display, employs the principle of emitting light by utilizing the electric discharge phenomenon of its surface. Accordingly, radiation of electromagnetic waves in the range of about 30 MHz to 130 MHz from the screen is not small, and a shield is necessary. A conventional electromagnetic wave shield adopts a method of attaching an EMI shield filter to the front surface of the screen.
As such an EMI shield filter, one that causes light transmission with the structure of a conductive mesh with a small gap is used. Other than the mesh structure, alternatively, an EMI shield filter as shown in Japanese Patent Laid-Open No. 9-247584 (reference 1) is used, on which a light-transmitting thin film is formed by sputtering a metal, e.g., silver or gold, or vacuum-depositing a metal, e.g., tin oxide, on the filter base.
When the display apparatus is a CRT (Cathode-Ray Tube), electromagnetic waves such as X-rays are radiated from the display screen. Conventionally, as an electromagnetic wave shield for reducing radiation, one in which a plate-like main body such as an iron foil having a large number of through holes is adhered to a light-transmitting film is known, as shown in Japanese Utility Model Laid-Open No. 62-109495 (reference 2).
FIG. 4 shows an electromagnetic wave leakage prevention filter described in reference 1, which employs a conductive mesh structure. The width of the conductors is set to 15 xcexcm, and the gaps between conductors in the vertical and horizontal directions that constitute the conductive mesh are both set to 127 xcexcm. This conductive mesh is fabricated by forming conductors by electroless-plating a metal such as copper, then electroless-plating a metal such as nickel on the plated conductors, and by etching the conductors.
As a conductive mesh, one can also be used which is formed by adhering metal fabric, obtained by electroless-plating copper or copper nickel to a synthetic resin mesh fabric, to a filter base.
With the filter of reference 1, the conductors themselves that constitute the mesh do not transmit light, but the mesh structure imparts light transmission properties to the filter. For this purpose, the mesh is obliquely tilted with an inclination of 45xc2x0, so mesh-is not overlaid on the rows and columns (horizontal and vertical directions) of pixels of the plasma display to obstruct the image.
Each of FIGS. 5A to 5D shows the electromagnetic wave shield filter described in reference 2 for mainly shielding X-rays radiated from the CRT. In the main body of this electromagnetic wave shield filter, through holes are formed in a copper or iron foil. The through holes can have various shapes, e.g., a circular shape (FIG. 5A), a hexagonal shape (FIG. 5B), a rectangular shape (FIG. 5C), and a slit shape (FIG. 5D).
Regarding the EMI shield filter with the mesh structure, if the same shield material is used, the smaller the mesh, the higher the shield performance, and the thicker the material, the higher the shield performance. Regarding the electromagnetic waves radiated from the display apparatus, since the EMI standards must be satisfied, an EMI shield filter having a conventional mesh structure uses a fine mesh structure at the cost of the light transmission properties.
Of the mesh structure having a conductor width of 15 xcexcm and a conductor-to-conductor distance of 127 xcexcm as shown in FIG. 4, when the transmittance (opening ratio) is calculated, it is ((127xe2x88x9215)xc3x97(127xe2x88x9215))/(127xc3x97127)=77.8 (%), that is, it is a low ratio of about 78%. When this electromagnetic wave leakage prevention filter is attached, the light transmittance is decreased 22%, and the brightness of the screen decreases largely. This means that to maintain a high screen brightness inversely, an excessively large power corresponding to an increase in transmittance of 22% must be supplied to the display apparatus.
In the conventional X-ray shield filters shown in FIGS. 5A to 5D, although the plurality of through holes formed in the plate-like filter main body have various types of shapes, these filters do not realize increases in both shield performance and transmittance realized by considering the characteristics of the display apparatus.
It is an object of the present invention to provide an EMI shield filter which aims at improving the light transmittance while the requirement as the EMI shield filter is met, and a display apparatus.
It is another object of the present invention to provide a display apparatus having an EMI shield filter which can realize predetermined brightness and decrease power consumption.
In order to achieve the above objects, according to the present invention, there is provided an EMI (ElectroMagnetic Interference) shield filter for a display unit, with a conductive mesh in which a plurality of first and second conductors are arranged in horizontal and vertical directions, wherein a gap in one of the first and second conductors is set larger than that in the other one of the first and second conductors that attenuate a field strength of higher one of horizontal and vertical polarized components of EMI of the display unit, so that the horizontal and vertical polarized components have substantially the same field strength after attenuation.